Electric arc furnaces (EAFs) make steel by using an electric arc to melt one or more charges of scrap metal, hot metal, iron based materials, or other meltable materials, which is placed within the furnace. Modern EAFs may also make steel by melting DRI (direct reduced iron) alone, or combined with the hot metal from a blast furnace. In addition to the electrical energy of the arc, chemical energy is provided by auxiliary burners using fuel and an oxidizing gas to produce combustion products with a high heat content to assist the melting.
If the EAF is used as a scrap melter, the scrap burden is charged by dumping it into the furnace through the roof opening from buckets, which also may include charged carbon and slag forming materials. A similar charging method using a ladle for the hot metal from a blast furnace may be used along with injection of the DRI by a lance to produce the burden. Additionally, these materials could be added through other openings in the furnace.
In the melting phase, the electric arc and burners melt the burden into a molten pool of metal, termed an iron carbon melt, which accumulates at the bottom or hearth of the furnace. Typically, after a flat bath has been formed by melting of all introduced burden, the electric arc furnace enters a refining and/or decarburization phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the iron carbon melt and rise to the surface as slag. During the heating of the iron carbon melt, it reaches the temperature and conditions when carbon in the melt combines with oxygen present in the bath to form carbon monoxide bubbles. Generally, flows of oxygen are blown into the bath with either lances or burner/lances to produce a decarburization of the bath by the oxidation of the carbon contained in the bath.
The resulting decarburization reduces the carbon content of the bath to a selected level. If an iron carbon melt is under 2% carbon it becomes steel. Except for operations using the hot metal from the Blast furnaces, the EAF steel making processes typically begin with burdens having less than 1% carbon. The carbon in the steel bath is continually reduced until it reaches the content desired for producing a specific grade of steel, down to less than 0.1% for low carbon steels.
The EAF steel making process is an extremely exacting process that involves the simultaneous management of a variety of different variables. The numerous variables of the melting process must be kept within certain tolerances throughout the process to ensure an accurate and efficient melt is conducted. For instance, chemical energy can be added at certain stages of the process to facilitate the progression of the melt. Additionally, other chemicals can be inserted into the furnace to alter the state of the melt to progress the steel making process and protect the furnace equipment. Furthermore, it is desirable to identify when the process is complete and the steel is ready for removal. The steps involved in the steel making process, including inserting additional chemical energy, inserting other chemical substances, determining the whether the steel is ready for tapping, and many other necessary tasks, require that the operator have a safe, effective, and efficient means by which to access the melt. Sometimes, means provided the operator for accessing the iron carbon melt are inefficient, detrimental to the progression of the steel making process, and provide insufficient safety precautions for the operator.
First, any opening in an EAF for receiving a lance or taking measurements must be kept clear of slag, which can prove particularly difficult for openings located relatively low in the furnace. This may be done with a constant flow of air through the opening, if this opening is relatively small. Larger openings require a significant volume of constant air flow, which is not preferable since it may be costly to provide such a high volume of air and because it may cool the furnace. Counteracting such cooling affects is also costly. In practice, the protection of openings by the flow of compressed air does not result in a clean, unplugged opening.
Second, the furnace operator may be required to inject a variety of chemicals (fluxes) into the furnace during the steel making process to ensure the melt is progressing appropriately. For example, chemicals such as, but not limited to, lime, carbon, oxygen, aluminum, and silicon, may be introduced into the bath to reduce impurities and alter the chemical composition of the steel. The added chemicals can aid in such processes as the carburization or decarburization of the melt or the creation of foamy slag to shield the electric arc and protect the furnace equipment. The conventional methods of insertion of chemicals also require the operator to expose openings in the furnace's roof or upper shell. The lower the position of the openings within a furnace, the more effort required to keep the opening clean. The current trend is to keep the ports, or openings, higher above the level of molten steel. This results in a significant loss of small particles of inserted chemicals through the fume evacuation system of EAFs.
Third, the furnace operator encounters many challenges with the conventional methods of determining whether the melt is ready for tapping. A furnace must reach very high temperatures to melt burden into molten metal. For example, scrap steel melts at approximately 2768° F. To achieve such high temperatures, steel making furnaces are generally fully enclosed with a minimal number of openings. Due to the negative pressures in the EAF, furnace openings may allow ambient air into the furnace and create a cold spots. Additionally, it is typically desirable to raise the temperature of the melt sufficiently above the melting point (typically to 2950° F.-3050° F.) to allow the melt to be transferred from the furnace to a desired location and further processed without prematurely solidifying.
Additionally, due to the high temperature and splashing of slag and molten metal, it is not practical to install a permanent temperature gauge in the furnace to monitor the temperature of the molten metal bath. Accordingly, steel makers typically use disposable thermocouples to check the liquid bath temperature. Disposable probes are typically mounted in cardboard sleeves that slide onto a steel probe pole, which has internal electrical contracts. The disposable probe transmits an electrical signal to the steel pole, which in turn transmits the signal to an electronic unit for interpretation. Additional probes may be used to determine the carbon content and dissolved oxygen levels in the molten metal. Various disposable temperature and chemical content probes are known in the art.
Typically, disposable probes are inserted into the furnace through the slag door. Unfortunately, there are several drawbacks to measuring the temperature through the open slag door. For example, when the door is open, a large amount of cold air can be drawn into the furnace. If the molten metal bath is below the desired temperature, the additional heat losses due to temperature probing will require more energy to be consumed to reach the target temperature.
Another draw back to measuring steel bath parameters through the slag door involves the process of inserting a probe into the liquid bath. Many years ago, probes were only introduced into the melt manually. This manual operation puts the operator at great risk of injury. Today, some steel plants and foundries still use this manual procedure because most alternative systems are very costly. Each year, operators are seriously injured or even killed while taking furnace measurements manually through the open slag door. These injuries typically occur when uncontrollable reactions occur in the furnace thereby causing injury to the operator.
These reactions are caused by rapid reaction of oxygen and carbon in the furnace. Oxygen is injected into the steel bath to remove or balance elements such as, but not limited to, sulfur, phosphorus, manganese, silicon, and carbon. Although carbon reacts quickly with oxygen, as the carbon concentration in the steel bath decreases below 0.10% by weight, the oxygen-carbon reaction slows down considerably, resulting in stratification of molten bath. In order to reduce carbon below 0.05% in the steel bath, the active or free oxygen level in the steel must be about 700 ppm. If any material such as slag or scrap were to fall from the walls of the furnace into the steel bath, an eruption will occur. The oxidizable elements in the slag or steel will react with the active oxygen in the steel bath and create, very quickly, a large amount of combustible gasses. These gases can erupt with enough force to throw flame, slag, and steel a great distance. In addition, when the combustible gases created in this reaction are exiting the furnace through the slag door, they rapidly combust with the air outside of the furnace thus increasing the intensity of the reaction.
Such reactions occur so quickly that it creates an explosive effect. Tragically, if such reactions occur while the slag door is open for a manual measurement, the slag boil can overflow the furnace and cause great harm to the operator. Now, many furnace operators use a large, and expensive, mobile device for inserting probes into the furnace. Since the slag door must remain clear for removing slag from the furnace, a dedicated temperature probe insertion tool can not be installed adjacent to the slag door. Rather, the device must either have a very long arm to reach through the slag door to the bath, or it must be mobile so that it can be moved out of the way of the door for other processes.
When the slag door is opened, any slag and metal trapped at the door opening must be cleared to allow insertion of the measurement probe. Clearing the door can be done with a large ram that pushes the slag and scrap out of the door opening and into the melt. Since any scrap trapped in the opening is pushed into the melt adjacent to the door, a probe inserted through the door can not easily measure the temperature of the melt. It is a typical practice in the industry to wait for this scrap to be melted before taking a measurement. This practice adds additional time to the melting phase, and therefore additional expense, to the steel making process.
There are other potential options available for insertion of the temperature probe, but each has significant drawbacks and is not typically used in the industry. First, an opening could be provided in the side wall of the furnace and a temperature probe could be inserted through this opening. Unfortunately, there is not a good location for providing such opening. If the opening were provided low in the furnace, close to the melt, it would become clogged with slag. Thus, the slag would need to be removed prior to insertion of the probe. Prior to the present invention, there was not a device available for easily and efficiently cleaning slag from such an opening. Cleaning the slag from the hole is an onerous task because the slag solidifies on the walls of the furnace and can become quite thick. Thus, it would be difficult to clean the slag from the opening and insert the temperature probe in an efficient manner.
Alternatively, the opening could be provided very high on the side wall of the furnace where it would be less likely to become clogged with slag. This solution is also not desirable because the access opening would be far from the melt. Thus, an exceptionally long probe pole would be needed to reach down into the melt. To operate this pole, a long and heavy structure adjacent to the furnace wall must be constructed. The location of this structure is limited by potential interference with the movement of the furnace roof and the scrap bucket during the charge of melting material.
Finally, an additional drawback relates to the utilization of different systems for the introduction of chemical energy into the EAF shell, such as burners, oxygen injectors, carbon injectors and others is the standard practice of modern EAF steel melting. The systems are located on different parts of the furnace's shell, or roof, and assist in scrap preheating, melting, steel decarburization and refining through the opening in the shells. These systems only function during certain phases of the process. During the remainder of the process they must be maintained in a protective mode to keep the ports clean, such as pilot flame, or for the non-flammable apparatus, compressed air or nitrogen flow. The lower these openings are located in a furnace, the more effort required to keep the openings clean. The pilot flame, nitrogen, or compressed air flow increase the cost of operations and require the consumption of additional energy to make steel.
Therefore, it would be advantageous to provide a method and apparatus for accessing the melt through an opening in the furnace.
Therefore, it would be advantageous to provide a method and apparatus for inserting a lance through an opening in the furnace.
Therefore, it would be advantageous to provide a method and apparatus for inserting chemicals through an opening in the furnace.
Therefore, it would be advantageous to provide a method and apparatus for injecting chemicals into a molten metal bath through an opening in the furnace close to the bath.
Therefore, it would be advantageous to provide a method and apparatus for measuring the temperature of a molten metal bath through an opening in the furnace, other than the slag door.
Additionally, it would be advantageous to provide a method and apparatus for injecting chemicals into a molten metal bath through a dedicated chemical injection aperture.
Additionally, it would be advantageous to provide a method and apparatus for keeping a dedicated chemical injection aperture clear of slag and debris without using a constant flow of air.
Additionally, it would be advantageous to provide a furnace with a dedicated chemical injection aperture.
Therefore, it would be advantageous to provide a method and apparatus for protecting the openings in a furnace shell and roofs without using extensive amount of fuel, oxygen, nitrogen, or compressed air.